1. Field of the Invention
The invention pertains to a method and system for reconditioning a heterogeneous collection of electronic components in a Fire Control Radar (FCR) high frequency, high voltage dual mode radar transmitter used in state-of-the-art military aircraft including the F-15, F-16, F-18 and B-1 bombers. More particularly the invention relates to a method and system for removing embedded moisture and absorbed moisture from previously repaired and repairable FCR APG-68 tactical radar units to increase their normal repaired operational life from a few hundred hours or less to an expected life of about 500 hours.
The novel method involves extensive drying without damaging the heterogeneous collection of electronic components in the FCR APG-68 tactical radar unit at temperatures between 40 and 105 degrees Celsius for periods of time from about 2 hours to 96 hours and preferably 4 to 48 hours when employing a vacuum pressure between 0.1 Torr and 10,000 milliTorr and preferably below 100 milliTorr and then sealing such electronic components or reassembling and filling the FCR APG-68 tactical radar unit with a dry gas within about 1 to 30 minutes and preferably less than 5 minutes after treatment and while the unit is still warm or above 50° C.
2. Description of Related Art Including Information Disclosed Under 37 C.F.R. 1.97 and 1.98
High power radar transmitters fail periodically in service and are returned to depots for repair. At the depots the sulfur hexafluoride (SF6) is removed from the high voltage high frequency power supply or high voltage section, which is enclosed in a sealed pressure vessel or the FCR APG-68 tactical radar dual mode transmitter. The pressure vessel is then opened and the electronic components within the high voltage section are exposed to the atmosphere of the shop while the failed component(s) are being located and replaced. The system is sometimes left open for days and even weeks. After reassembly the high voltage electronic package is sealed into the pressure vessel, which is then evacuated, heated and dried under vacuum and refilled with sulfur hexafluoride (SF6). After being tested the transmitter is returned to service. It has been discovered by the inventors that the prior art evacuation heating and drying procedures removed only superficial moisture.
One of the problems not recognized in the prior art is that ground testing did not simulate long period testing under actual temperature conditions encountered in flight operations. Ground testing, while adequate for demonstrating operability of the reassembled unit, did not include actual operational conditions where high ground temperatures followed by rapid low temperature flight conditions resulted in changes in vapor pressure inside the sealed unit that caused two types of moisture, absorbed moisture and embedded moisture left in the unit to reduce the life of the FCR APG-68 unit in service.
The best known prior art involves the original manufacture of the FCR APG-68 dual mode transmitters. In the original manufacture of the transmitters the partially assembled electronic assemblies (FIG. 6) were tested for corona discharge and other electrical characteristics while immersed in baths of Fluorinert™. After assembly they were evacuated in a pressure vessel to remove air so that they could be filled with sulfur hexafluoride (SF6). Fluorinert™ evaporates without leaving any residue, but has a high boiling point and not all of it evaporates immediately. Originally Fluorinert™ residues contaminated the oil of vacuum pumps used for the subsequent evacuation and interfered with reaching a vacuum level in the milliTorr range. This problem was eliminated by adding the step of vacuum baking the electronic assemblies to remove Fluorinert™ prior to evacuation and filling with SF6.
As manufactured relatively early in the FCR APG-68 program, the High Voltage, High Frequency Power Supply unit shown in FIG. 6 was vacuum-baked in an inverted position for about two hours, then its cold plate (FIG. 6, 62 and FIG. 5, 62) was sealed to the aluminum high voltage pressure vessel (FIG. 5) by means of an O-ring located just inside of the bolt holes (FIG. 3, 46). The closing and sealing was carried out while the assembly was still warm and was surrounded by an atmosphere consisting largely of nitrogen. This open vacuum-baking process unknowingly and unwittingly removed a lot of the moisture originally present in the components and absorbed during initial manufacture. Once repaired any moisture left at the time of original manufacture combined with the moisture absorbed from the atmosphere during the current repair which also added to the moisture adsorbed during previous repair operations to form harmful absorbed and embedded moisture that resulted in increasing mean time between failure (MTBF) rates.
In the prior art repair process Fire Control Radar FCR APG-68 units are repaired and a final process performed on repaired transmitters is to evacuate them through a Schrader valve while they are being heated and then to backfill the high voltage high frequency power supply with SF6. The vacuum is drawn through passages in the Schrader valve that are only about 0.060 inch in diameter and whose conductance is, therefore, very low. As a result it is believed that only a small amount of moisture and possibly only the moisture already in the air within the pressure vessel is removed at the time of evacuation and heating. The bulk of the moisture that has been absorbed from the atmosphere in the shop during the repair process remains embedded in the various electronic components, largely in organic insulating materials and builds up as embedded moisture as a consequence of repeated repairs.
Over the last twenty years the mean-time-between-failure (MTBF) of the transmitters has been falling from over 500 hours of operational life to values in the low hundreds of hours. Frequently transmitters now fail after only a few tens of hours of operation after having been serviced and ground tested. Many of the FCR APG-68 units have therefore been repaired dozens of times with each repair likely adding to the total moisture embedded in the high voltage high frequency power supply.
The best known prior art which was employed during the original manufacturing process did not have as its primary purpose the removal of moisture and did not specifically quantitatively test for moisture removed. The prior art process of removing Fluorinert™ is believed to have unwittingly and unknowingly removed much of the moisture absorbed during the manufacturing process, leaving a quantity of tolerable moisture. This tolerable moisture included intrinsic moisture that could not be removed without removing volatile organic plasticizers and organic materials. This tolerable moisture and intrinsic moisture did not significantly impair the normal expected 500 hour MTBF rate. The standard practice of heating and evacuation through the Schrader valve at vacuums typically of 2 Torr does not remove the bulk of the moisture absorbed during the immediately preceding repair operation and is believed not to remove embedded moisture or moisture that was absorbed during previous repair operations.
The invidious nature of the absorbed and embedded moisture in the high voltage high frequency power supply was first recognized by the inventors after discovering the surprising amount of water removed from a high voltage high frequency power supply from an FCR APG-68 defective unit from a B-1 bomber as will be described hereinafter in greater detail. The amount of water removed as moisture is believed to have been deeply embedded in the organic components of the high voltage high frequency power supply. On the ground at a constant temperature the moisture content of the vapor space in the pressure vessel approaches an equilibrium with the moisture content of the organic and inorganic solid state materials in the high voltage high frequency power supply.
The time between flights would allow this equilibrium to be approached at sometimes high ground temperatures. However the rapid change in temperature encountered in flight level altitudes which change at about 1.4 degrees Centigrade per 1,000 feet can drop temperatures by 15° C. in about 10 seconds. Such a rapid cooling due to a rapid change in altitude results in a rapid rise in the relative humidity in the sealed high voltage high pressure vessel. As the relative humidity in the pressure vessel rises rapidly and exceeds 100% condensation would occur resulting in arcing, partial discharges and failure of the FCR APG-68 dual mode transmitter.
The deleterious effect of moisture on the electrical components and properties of insulators is well known. Camilli U.S. Pat. No. 2,300,910 refers to the vacuum treatment and drying to remove all moisture prior to the impregnation of the paper insulation in high voltage windings of transformers during their manufacture. Many methods have been proposed for the drying of electronic components during manufacture such as Wennerstrum U.S. Pat. No. 4,882,851 which discloses the use of microwave heating. Microwave heating cannot be applied to an assembled FCR APG-68 tactical radar dual mode transmitter. Other prior art such as Schroder U.S. Pat. No. 5,189,581 discloses use of a desiccant for removing moisture from the housing of a videocassette recorder.
Leech U.S. Pat. No. 5,433,020 discloses use of a cold trap with a valve between vacuum pump and trap to maintain a fixed differential pressure to control flow rate during the vacuum drying of an object. In contrast the system of the invention employs a valve between cold trap and vacuum chamber to permit measurement of the rate of evolution of embedded and absorbed moisture.
Schober U.S. Pat. No. 3,792,528 dries windings of high voltage transformers, seals them, washes out the sealant and dries the transformer with kerosene vapor before filling with transformer oil. Kerosene vapor cannot be employed to dry FCR APG-68 tactical radar transmitters because of the difficulty in complete removal of the kerosene prior to filling with SF6.
Inoue Tamotsu JP 61 174 707 improves the dielectric strength of the gas of a gas-filled transformer by intermittently circulating the gas through an external drier. This is not practical in an air-borne FCR APG-68 dual mode radar transmitter because the length of time required is so much greater than through the use of vacuum.
Michio, et al. JP 1110 2829 reduces the rate at which paper insulation deteriorates by heating electrical equipment under vacuum by passing current through the windings. This method of heating the windings is not practical for radar components within the high voltage section, which involve many different components other than transformer windings. Similarly Gmeiner Paul (DE 19 501 323) dries transformers and treats the oil by heating with current through the coils.
Boguslaysky US 2003 0183929 thermally conditions components on IV packages before and/or after repairing them in order to prevent moisture from damaging the packages when subsequently subjected to soldering temperatures. The need to maintain dryness of electrical packages that will be exposed to soldering temperatures for purposes of soldering is very different from removing moisture from FCR APG-68 radar transmitter units to increase their operational life. Dias U.S. Pat. No. 4,347,671 dries the interior of metal surfaces such as tubing for high purity gases by passing through a reactive gas, such a procedure would damage the components of a high voltage high frequency power supply.
The premature failures of repaired FCR APG-68 units have resulted in extensive investigations in the prior art. Arcing and partial discharge and failure have been attributed to the contamination of Coolanol™ which is used as a circulating coolant for the FCR APG-68 tactical radar unit as well as to the contamination of the sulfur hexafluoride gas in the high voltage high frequency power supply.
It has been found by the inventors that failed FCR APG-68 tactical radar units contain contaminated Coolanol™ 25 exhibiting increased color, odor and viscosity and decreased resistivity and in extreme cases sludge. This sludge can be deposited on the heat exchanger surfaces or in the traveling wave tube (TWT). As a result the heat transfer coefficient and the flow rate can decrease because of the formation of solid contaminants that raise the temperature of the TWT, which accelerates the decomposition of the Coolanol™ 25 and the eventual malfunction of the FCR APG-68 tactical radar unit.
Those skilled in the art of FCR APG-68 tactical radar units have extensively investigated Coolanol™ 25 as a source of the problems of arcing, the creation of hot spots and the failure of FCR APG-68 tactical radar units. One study involved the replacement of Coolanol™ 25 with polyalphaolefin under the title Coolanol 25R Replacement for Military Aircraft Cooling Systems AF06-083 which contract was awarded to METSS Corporation of Westerville, Ohio and an Article entitled Methodology for Comparison of Hydraulic and Thermal Performance of Alternative Heat Transfer Fluids in Complex Systems, By Ghajar, Tang and Beam, Vol. 16, Issue 1 January-March 1995 Heat Transfer Engineering. 
Those skilled in the art have also investigated the FCR APG-68 tactical radar unit as a function of the purity of sulfur hexafluoride (SF6) or its contamination. SF6 purity is important since the electronics package of the high voltage high frequency unit is sealed in an atmosphere of SF6. There is however disagreement in the literature on the effect of moisture on the behavior of SF6 in arcing and corona discharge.
As a result those skilled in the art have considered various options to remedy the premature ageing and high rate of failure of FCR APG-68 tactical radar units. The initial cost of acquisition at almost one million dollars a unit and their reduced service life and requirements for repair and maintenance have provided a great incentive for finding an acceptable method or procedure for remediating and upgrading the performance of these vital tactical radar units.